In case of an electric train, during ordinary braking, electrical braking based on regenerative braking is performed to convert the inertial energy of a vehicle into electric energy, and to restore the regenerative energy to an overhead line, that is, a DC power feed line. In the regenerative braking, it is indispensable that the side of the DC power feed line has a regenerativity, or that another electric train under power running as acts as a regenerative load exists on the DC power feed line side.
Here, in a case where the regenerative energy of the electric train in the regenerative braking state is greater than power which is consumed by the electric train in the power running state, the voltage of the overhead line or the voltage of a filter capacitor disposed at a stage preceding an inverter increases to an overvoltage, and a protective function is sometimes actuated. As means for improving this problem, it is common practice that the regenerative power from the electric train in the regenerative braking state is fined to lower a braking force based on the regenerative braking, and that the lowered component of the braking force is compensated by mechanical braking. In this case, however, a brake shoe wears off due to the mechanical braking, and hence, the maintenance of the brake shoe needs to be performed in a certain fixed period.
In order to avoid the wear of the brake shoe, it has been known that a brake chopper which consists of a switching element and a resistor is disposed on the DC side of the inverter. In this case, when the regenerative power based on the electrical braking is great to raise the DC output voltage of the inverter, the brake chopper is operated to consume the regenerative power. Since, however, the brake chopper is added to a control apparatus in this case, there has been the problem that the control apparatus becomes large in size and high in fabrication cost.
In FIG. 1 of JP2003-199204A (Patent Document 1), it is disclosed that an electric-double-layer capacitor which can store regenerative energy is employed instead of the brake chopper. In this patent document 1, in a control apparatus for an electric train, wherein a smoothing capacitor is disposed in a DC power feed circuit which is connected to a DC power feed line through a collector shoe and wherein a DC voltage smoothed by the smoothing capacitor is fed to a variable-voltage variable-frequency inverter, the electric-double-layer capacitor is connected in parallel with the smoothing capacitor through a DC/DC converter having a switching element, and in a regenerative braking state, regenerative energy from the inverter is stored in the electric-double-layer capacitor, whereas in a power running state or a coasting running state, the energy stored in the electric-double-layer capacitor is fed to the power feed line or the inverter.
Also, in FIG. 10 of JP2004-104976A (Patent Document 2), it is disclosed that an electric-double-layer capacitor is employed instead of a brake chopper circuit for the purpose of storing regenerative energy.
Patent Document 1: JP2003-199204A (Claim 1, FIG. 1)
Patent Document 2: JP2004-104976A (Sector 0072, FIG. 10)